The objective of this proposal is to determine the mechanism of regulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) of estrogen dependent human breast cancer tumor growth in mice. Ultimately, knowledge of this mechanism will provide: 1) the basis of the observed drug resistance to antiestrogen therapy for estrogen dependent breast cancer, 2) and approach for mechanism-based endocrine related risk assessment associated with chronic low level exposures to dioxin-like xenobiotics, and 3) novel Ah locus-mediated chemotherapeutic strategy for steroid hormone-dependent cancer and understanding of the role of estrogen and estrogen receptors in breast cancer etiology. TCDD inhibits the 17beta-estradiol (E2)-dependent postconfluent proliferation of MCF-7 human breast cancer cells in vitro. This effect is elicited though an Ah locus-mediated process which is coincident with depletion of E2 by increased metabolism. In vivo studies demonstrated a similar, but transient suppression of E2-dependent human breast cancer MCF-7 tumor growth in immunosuppressed male BDF1 mice. There was a two-week period of TCDD sensitivity, in which TCDD suppresses E2 stimulation of MCF-7 tumor growth, followed by a TCDD-resistant phase, during which the tumor growth rate is similar to that in non TCDD-treated controls. The conversion of this estrogenic effect from TCDD sensitive to TCDD resistant offers a unique opportunity to examine changes in a human estrogen-regulated tissue in vivo, for further elucidation of the mechanism of TCDD mediated antiestrogenicity. It was also determined that E2 depletion results in increased levels of MCF-7 estrogen receptor. This suggests the hypothesis that the sensitivity and subsequent resistance to TCDD suppression of E2-dependent MCF-7 human breast tumor growth in vivo is initially due to TCDD-dependent, Ah locus mediated estrogen depletion in the tumor with subsequent enhancement of tumor sensitivity to E2 resulting from a compensatory increase in estrogen receptor levels. The following specific aims are proposed to test this hypothesis: 1) Determine if there is a requirement for Ah locus function in the host or tumor for sensitivity and resistance to TCDD suppression of E2-dependent MCF-7 tumor growth by use of mice and MCF-7 cells of various Ah locus competency. 2) Determine if the TCDD suppression of E2-dependent MCF-7 tumor growth is associated with E2 depletion by induced metabolism by measurement of E2 and E2 metabolism. 3) Determine if the estrogen receptor status of the TCDD resistant tumor is altered compared to non TCDD-treated tumors by measurement of estrogen receptor function, abundance, and synthesis.